As a heat exchanger of this type, Patent Document 1 discloses a sub-cool type condenser of which a heat exchanger core is constituted by a condenser section for condensing a refrigerant and a sub-cooler section for supercooling the refrigerant of liquid phase obtained by passing the refrigerant from the condenser section to a receiver tank so as to subject the refrigerant to gas-liquid separation, and in which an upper header tank and a lower header tank are connected to each other by a plurality of vertically extending tubes and each have the interior divided by a partition plate into a condenser header part associated with the condenser section and a sub-cooler header part associated with the sub-cooler section.